var crypto = require("crypto");

function WXBizDataCrypt(appId, sessionKey) {
  this.appId = appId;
  this.sessionKey = sessionKey;
}

WXBizDataCrypt.prototype.decryptData = function (encryptedData, iv) {
  // base64 decode
  var sessionKey = new Buffer(this.sessionKey, "base64");
  encryptedData = new Buffer(encryptedData, "base64");
  iv = new Buffer(iv, "base64");

  try {
    // 解密
    var decipher = crypto.createDecipheriv("aes-128-cbc", sessionKey, iv);
    // 设置自动 padding 为 true，删除填充补位
    decipher.setAutoPadding(true);
    var decoded = decipher.update(encryptedData, "binary", "utf8");
    decoded += decipher.final("utf8");

    decoded = JSON.parse(decoded);
  } catch (err) {
    throw new Error("Illegal Buffer");
  }

  if (decoded.watermark.appid !== this.appId) {
    throw new Error("Illegal Buffer");
  }

  return decoded;
};

module.exports = WXBizDataCrypt;